Chapter 21

Question 1

What is a buffer solution?

Answer 1

A system that minimises pH changes when small amounts of an acid/base is added

Question 2

What do buffer solutions contain?

Answer 2

1) weak acid

2) conjugate base

Question 3

What does the weak acid, HA in a buffer solution do?

Answer 3

removes added alkali

Question 4

What does the conjugate base, A⁻ in a buffer solution do?

Answer 4

removes added acid

Question 5

What happens when alkalis and acids are added to a buffer?

Answer 5

1) two components in buffer solution react + are used up

2) solution loses buffering ability towards added acid/alkali when one component has fully reacted

Question 6

Does the pH of a buffer solution change by a small/big amount?

Answer 6

small amount

Question 7

What are the 2 ways for preparing weak acid buffer solutions?

Answer 7

1) preparation from a weak acid and its salt

2) preparation by partial neutralisation of the weak acid

Question 8

How can a buffer solution be prepared from a weak acid and its salt?

Answer 8

1) mix a solution of carboxylic acid with a solution of one of its salts
2) when the carboxylic acid is added to water, the acid partially dissolves and the amount of carboxylate ions in solution is very small

Question 9

What types of compounds are salts of weak acids?

Answer 9

ionic compounds

Question 10

What happens when the salt is added to water?

Answer 10

it completely dissolves

Question 11

salt of weak acid + water –>

Answer 11

carboxylate ion + salt ion

Question 12

How can a buffer solution be prepared by partial neutralisation of the weak acid?

Answer 12

1) adding an aqueous solution of an alkali to an excess of the weak acid
2) weak acid is partially neutralised by alkali, forming conjugate base
3) some weak acid is left unreacted

Question 13

When preparing a buffer solution by partial neutralisation of the weak acid, what does the resulting mixture contain?

Answer 13

1) salt of the weak acid

2) any unreacted weak acid

Question 14

What controls the pH in an acid buffer solution?

Answer 14

the conjugate acid-base pair, HA (aq)/A⁻ (aq)

Question 15

How does the conjugate base remove added acid?

Answer 15

1) [H⁺(aq)] increases
2) H⁺(aq) ions react with conjugate base, A-(aq)
3) equilibrium position shifts to left, removing most of H⁺ ions

Question 16

How does the weak acid remove added alkali?

Answer 16

1) [OH⁻(aq)] increases
2) small concentration of H⁺(aq) ions reacts with OH⁻(aq) ions:
H⁺(aq) + OH⁻(aq) –> H₂O(l)

Question 17

What is different about the buffer solutions of different weak acids?

Answer 17

buffer solutions operate over different pH ranges

Question 18

When is a buffer most effective at removing either added acid/alkali?

Answer 18

when there are equal concentrations of the weak acid and conjugate bae

Question 19

When [HA(aq)] = [A⁻(aq)]:

Answer 19

1) pH of buffer solution is the same as pKₐ value of HA

2) operating pH is typically over 2 pH units, centred at pH of pKₐ value

Question 20

How can the pH of the buffer solution be fine-tuned?

Answer 20

by adjusting the ration of the concentrations of weak acid and conjugate base

Question 21

What is the equilibrium expression for a weak acid?

Answer 21

HA(aq) ⇌ H⁺(aq) +A⁻(aq)

Question 22

What is the Kₐ expression for a weak acid?

Answer 22

[H⁺(aq)] [A⁻(aq)] / [HA(aq)]

Question 23

When calculating the pH of a weak acid, what approximation do you make?

Answer 23

[H⁺(aq)] = [A⁻(aq)}

Question 24

Why is the approximation [H⁺(aq)] = [A⁻(aq)} not true for a buffer solution?

Answer 24

A⁻(aq) has been added as one of the components of the buffer

Question 25

What pH does blood plasma need to be maintained between?

Answer 25

7.35-7.45

Question 26

What is the pH of blood plasma controlled by?

Answer 26

1) a mixture of buffers

2) carbonic acid-hydrogencarbonate buffer (H₂CO₃/HCO₃⁻)

Question 27

What should the pH of normal healthy blood be?

Answer 27

7.40

Question 28

What condition develops when the pH of blood plasma falls below 7.35?

Answer 28

acidosis

Question 29

What are the symptoms of acidosis?

Answer 29

1) fatigue
2) shortness of breath
3) shock
4) death

Question 30

What condition develops when the pH of blood plasma rises above 7.45?

Answer 30

alkalosis

Question 31

What are the symptoms of alkalosis?

Answer 31

1) muscle spasms
2) light-headedness
3) nausea

Question 32

What happens to the carbonic acid-hydrogencarbonate bufferon addition of an acid, H⁺(aq)?

Answer 32

1) [H⁺(aq)] increases
2) H⁺(aq) ions react with conjugate base, HCO₃⁻(aq)
3) equilibrium position shifts to left, removing most H⁺(aq) ions

Question 33

What happens to the carbonic acid-hydrogencarbonate bufferon addition of an alkali, OH⁻(aq)?

Answer 33

1) [OH⁻(aq)] increases
2) small concentration of H⁺(aq) ions reacts with OH⁻(aq) ions:
H⁺(aq) + OH⁻(aq) –> H₂O(l)
3) H₂CO₃ dissociates, shifting equilibrium position to right to restore most H⁺(aq) ions

Question 34

Does the body produce more acidic/alkaline materials?

Answer 34

acidic

Question 35

What do acid-base titrations use to monitor neutralisation reactions accrately?

Answer 35

indicators

Question 36

What are the key features of a pH titration curve?

Answer 36

1) excess of acid - pH increases slowly as basic solution is added
2) equivalence point - centre of vertical section of pH titration curve
2) vertical section - pH increases rapidly on addition of small volume of base. acid and base concentrations similar
3) excess of base - pH increases slowly as basic solution is added

Question 37

What is the equivalence point?

Answer 37

the volume of one solution that exactly reacts with the volume of the other solution

Question 38

Is an acid-base indicator a strong or weak acid?

Answer 38

weak acid, HA, that has a distinctively different colour from its conjugate base, A⁻

Question 39

What does the sensitivity of an indicator depend upon?

Answer 39

1) indicator itself

2) eyesight

Question 40

What indicator should you use in a titration?

Answer 40

one that has a colour change which coincides with the vertical section of the pH titration curve

Question 41

What is the end point of a titration?

Answer 41

when the indicator contains equal concentrations of HA and A⁻, and the colour will be in between the two extreme colours

Question 42

For which titration is no indicator suitable?

Answer 42

weak acid-weak base

Question 43

Why is no indicator suitable for a weak acid-weak base titration?

Answer 43

there is no vertical section and, even at its steepest, the pH requires several cm³ to pass through a typical pH indicator of 2 pH units